Accessing "Public" methods from "Private" methods in javascript class

Question

Is there a way to call "public" javascript functions from "private" ones within a class?

Check out the class below:

function Class()
{
    this.publicMethod = function()
    {
        alert("hello");
    }

    privateMethod = function()
    {
        publicMethod();
    }

    this.test = function()
    {
        privateMethod();
    }
}

Here is the code I run:

var class = new Class();
class.test();

Firebug gives this error:

publicMethod is not defined: [Break on this error] publicMethod();

Is there some other way to call publicMethod() within privateMethod() without accessing the global class variable [i.e. class.publicMethod()]?

I hope it's just a typo in your example here, but your privateMethod is a global variable. — Jimmy
– Jimmy, Commented Apr 24, 2010 at 7:34
In Firefox if I call class.privateMethod() there is an error whereas class.publicMethod() works fine, so it seems like there is some difference between the two functions. — mon4goos
– mon4goos, Commented Apr 24, 2010 at 7:56

gnarf · Accepted Answer · 2010-08-13 09:20:14Z

8

You can save off a variable in the scope of the constructor to hold a reference to this.

Please Note: In your example, you left out var before privateMethod = function() making that privateMethod global. I have updated the solution here:

function Class()
{
  // store this for later.
  var self = this;
  this.publicMethod = function()
  {
    alert("hello");
  }

  var privateMethod = function()
  {
    // call the method on the version we saved in the constructor
    self.publicMethod();
  }

  this.test = function()
  {
    privateMethod();
  }
}

edited Aug 13, 2010 at 9:20

answered Apr 24, 2010 at 7:18

gnarf

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Comments

user663031 · Accepted Answer · 2013-06-17 04:15:20Z

5

The accepted answer has the possibly undesirable side effect that separate copies of publicMethod, test, and privateMethod will be created in each instance. The idiom for avoiding this is:

function Class()
{}

Class.prototype=(function()
{
    var privateMethod = function(self)
    {
        self.publicMethod();
    }


    return 
    {
        publicMethod: function()
        {
            alert("hello");
        },
        test: function()
        {
            privateMethod(this);
        }
    };
}());

In other words, you need to pass the this to the private function as an argument. In return, you get a true prototype without having to pollute each instance with its own versions of the private and public functions.

answered Jun 17, 2013 at 4:15

user663031

1 Comment

gnarf Over a year ago

Nice! Rather than pass in, you could always privateMethod.call(this) too

SaganRitual · Accepted Answer · 2013-07-31 19:44:12Z

2

torazaburo's answer is the best one, as it avoids creation of multiple copies of the private members. I'm surprised that Crockford doesn't mention it at all. Alternately, depending on the syntax you prefer for declaring public member functions, you could do this:

function Class()
{}

(function() {
    var privateMethod = function(self) {
        self.publicMethod();
    };

    Class.prototype.publicMethod = function() {
        alert('hello');
    };

    Class.prototype.test = function() {
        privateMethod(this);
    };
}());

answered Jul 31, 2013 at 19:44

SaganRitual

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Comments

Ninja · Accepted Answer · 2014-04-02 07:25:31Z

0

Is this approach not a advisable one? I am not sure though

var klass = function(){
  var privateMethod = function(){
    this.publicMethod1();
  }.bind(this);

  this.publicMethod1 = function(){
    console.log("public method called through private method");
  }

  this.publicMethod2 = function(){
    privateMethod();
  }
}

var klassObj = new klass();
klassObj.publicMethod2();

answered Apr 2, 2014 at 7:25

Ninja

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